In addition to its high melting points, excellent electrical characteristics and mechanical properties, boron nitride has such preferable properties that it is hardly attacked by oxygen, chemicals or the like, and hence products of boron nitride such as fibers, films, moldings and the like now attract keen interests of various fields of industry. The products are used for various purposes, for example, reinforcing materials for composite materials, diaphragm for fused salt cells, electronic parts and the like.
Because of the high melting point of boron nitride, however, it is difficult to prepare fibers, films or moldings directly from boron nitride by the fusion thereof. Under such circumstances, known processes for obtaining fibers, films or moldings of boron nitride generally include a process by sintering fibers, films or moldings which are previously prepared from organoboron nitride polymers and a process by using organoboron nitride polymers as binders for boron nitride powders to prepare films or various types of moldings comprising a mixture of boron nitride powders and organoboron nitride polymers, followed by sintering.
As processes for preparing such organoboron nitride polymers suitable for use in forming fibers, films or moldings thereof or for use as binders for boron nitride powders as mentioned above, there has heretofore been known only a process in which borazine derivatives are singly subjected to heat polymerization. In the practice of this process, however, there was often observed formation of cross-linked polymers which were infusible or insoluble, and it was relatively difficult to produce excellent polymers for use in forming the above-mentioned fibers, films or moldings thereof or for use as binders for boron nitride powders.